Sigma lens issue?

I have a rebel xt and purchased a Sigma DC 18-125 1:3.5-5.6 as I didn't have the money for the Canon lens I wanted. I am having persistant focus issues, almost as if the focal point isn't in the center of the lens but towards the bottom or even edges. If anything is in the perifary (sp?) of my subject it focuses on that.

I shoot my son most of all and manual focus on a 5 year old boy is difficult tho I suppose I could try it, even when there are no things in the way I get fuzzy pics as well. Was wondering if anyone had an similar experience or suggestions?

Int he photo below the subject is fuzzy (the ride was not moving at all) but the car they are sitting in appears sharp. This photo was resized but not cropped.

I also have the same lens and have had a similar problem. When I tried to use my 350D in auto mode the focus was rubbish, but when I used AV with AI Servo (for motorsport photos ) it works fine.

it works fine for all situations, or just for fast outdoor shooting?

Here is a photo taken with my lens

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Have you set the camera for focusing with the centre focus point or is it selecting it's own focus point according to whatever is closest? This makes a big difference.

It is set to AI focus